Stopping chain rubbing on cage

Alibran

It's time for the stupid question of the week ....

I need to adjust my front derailleur to stop the chain rubbing against the cage in the very lowest gear (small ring, large sprocket). I know I need to turn the low limit screw.

How do I know which one this is? (I can't see any H and L markings.) And which way do I need to turn it? (It would also be nice to know how far because I don't want to start the chain jumping off the small chain ring, but I suspect the answer to that is, "How long is a piece of string?")

My chainset was originally shimano sora triple, but was changed to 9-speed tiagra. I think it was only the rear cassette and the shifters that were changed, so the chain rings and the front derailleur are original. The 2 limit screws are positioned one above the other, but I suspect it isn't as simple as assuming the bottom one is the low limit screw.

I feel embarrassed even having to ask this, but since my mechanical ability is best described as incompetent, and any time I touch the gears on a bike, I usually break them, I'd like to try and get it right this time.

kettrinboy

Theres an easy way to tell which screw is which, when you have the FD on its fully inward position look at the limit screws, one should be touching the FD and the other one shouldnt, you adjust the one thats touching,you might need to turn the bike upside down and use a torch to see them depending on the FD, turning the screw anti-clockwise will move the FD inwards,when youve adjusted it a bit, spin the pedals and change the FD from small to big ring and back to make sure it doesnt overshift and send the chain onto the bb shell if you havnt got a chain catcher, just a fraction of a turn at a time until its right.

benno68

I'm pretty sure you will need to adjust the inner screw (closest to the frame) turning it anti clockwise to loosen it a touch, may only need a quarter turn if that.

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/TIAGRA/FD-4500/SI_5K70A_En_v1_m56577569830606969.pdf

Courtesy of Shimano:

1. Low adjustment
Set so that the clearance between the chain guide inner
plate and the chain is 0 - 0.5 mm.

You should be able to see the cage move as you turn the screw.

Hope this helps.

Secteur

Remember that in the "gears you shouldn't use" it may rub, even with a perfect set up.

e.g. I get no rub on the gears I use, but if I put it on the large ring and the largest sprocket, it will rub (but I would never use this combination on the road, so it doesnt matter).

Alibran

Thanks for the tips, everyone. I think I've sorted it now. It only took 1/8 of a turn.

Secteur, I never cross my chain, and I did say in my first post "small ring, large sprocket".